Transversely-excited film bulk acoustic resonator using pre-formed cavities

ABSTRACT

Acoustic resonator devices and filters are disclosed. An acoustic resonator device includes a substrate having a surface, wherein the surface comprises an etched cavity, and a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans the cavity. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate, wherein interleaved fingers of the IDT are aligned with the cavity such that the interleaved fingers are disposed on the diaphragm. The IDT is configured to excite a primary acoustic mode in the diaphragm in response to a radio frequency signal applied to the IDT.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application No.62/873,726, filed Jul. 12, 2019, and provisional application No.62/873,732, filed Jul. 12, 2019, the entire contents of both of whichare incorporated herein by reference.

This patent is also a continuation-in-part of application Ser. No.16/438,121, filed Jun. 11, 2019, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, which is a continuation-in-part of application Ser.No. 16/230,443, now U.S. Pat. No. 10,491,192 B2, filed Dec. 21, 2018,entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, which claimspriority from the following provisional patent applications: application62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 3C is an alternative schematic plan view of an XBAR

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5A is a schematic cross-sectional view of a substrate for an XBAR.

FIG. 5B is a schematic cross-sectional view of a piezoelectric substratefor forming a piezoelectric plate of an XBAR.

FIG. 5C is a schematic cross-sectional view of the substrate of FIG. 5Abonded to the piezoelectric substrate of FIG. 5B.

FIG. 5D is a schematic cross-sectional view of the device of FIG. 5Cafter the piezoelectric plate has been separated from the piezoelectricsubstrate.

FIG. 5E is a schematic cross-sectional view of the device of FIG. 5Dafter interdigital transducer formation.

FIG. 6 is a schematic circuit diagram and layout of a filter usingXBARs.

FIG. 7 is a flow chart of a process for fabricating an XBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3A and FIG. 3B). The cavity 140 maybe formed, for example, by selective etching of the substrate 120 beforeor after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE′ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness ts of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be aluminum, a substantially aluminum alloys,copper, a substantially copper alloys, beryllium, gold, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of other metals, such as chromium or titanium, may beformed under and/or over the fingers to improve adhesion between thefingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In FIG. 3A, a piezoelectric plate310 is attached to a substrate 320. A portion of the piezoelectric plate310 forms a diaphragm 315 spanning a cavity 340 in the substrate. Thecavity 340 does not fully penetrate the substrate 320. Fingers of an IDTare disposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may contiguous with the restof the piezoelectric plate 310 around a large portion of a perimeter 345of the cavity 340. For example, the diaphragm 315 may contiguous withthe rest of the piezoelectric plate 310 around at least 50% of theperimeter 345 of the cavity 340.

In FIG. 3B, the substrate 320 includes a base 322 and an intermediatelayer 324 disposed between the piezoelectric plate 310 and the base 322.For example, the base 322 may be silicon and the intermediate layer 324may be silicon dioxide or silicon nitride or some other material. Aportion of the piezoelectric plate 310 forms a diaphragm 315 spanning acavity 340 in the intermediate layer 324. Fingers of an IDT are disposedon the diaphragm 315. The cavity 340 may be formed, for example, byetching the intermediate layer 324 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching theintermediate layer 324 with a selective etchant that reaches thesubstrate through one or more openings provided in the piezoelectricplate 310. In this case, the diaphragm 315 may contiguous with the restof the piezoelectric plate 310 around a large portion of a perimeter 345of the cavity 340. For example, the diaphragm 315 may contiguous withthe rest of the piezoelectric plate 310 around at least 50% of theperimeter 345 of the cavity 340. Although not shown in FIG. 3B, a cavityformed in the intermediate layer 324 may extend into the base 322.

FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350includes an IDT formed on a piezoelectric plate 310. A portion of thepiezoelectric plate 310 forms a diaphragm spanning a cavity in asubstrate. In this example, the perimeter 345 of the cavity has anirregular polygon shape such that none of the edges of the cavity areparallel, nor are they parallel to the conductors of the IDT. A cavitymay have a different shape with straight or curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. An RF voltage is applied to the interleaved fingers 430. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a shear-mode acoustic mode, inthe piezoelectric plate 410. In this context, “shear deformation” isdefined as deformation in which parallel planes in a material remainparallel and maintain a constant distance while translating relative toeach other. A “shear acoustic mode” is defined as an acoustic vibrationmode in a medium that results in shear deformation of the medium. Theshear deformations in the XBAR 400 are represented by the curves 460,with the adjacent small arrows providing a schematic indication of thedirection and magnitude of atomic motion. The degree of atomic motion,as well as the thickness of the piezoelectric plate 410, have beengreatly exaggerated for ease of visualization. While the atomic motionsare predominantly lateral (i.e. horizontal as shown in FIG. 4), thedirection of acoustic energy flow of the excited primary shear acousticmode is substantially orthogonal to the surface of the piezoelectricplate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no electric field immediatelyunder the IDT fingers 430, and thus acoustic modes are only minimallyexcited in the regions 470 under the fingers. There may be evanescentacoustic motions in these regions. Since acoustic vibrations are notexcited under the IDT fingers 430, the acoustic energy coupled to theIDT fingers 430 is low (for example compared to the fingers of an IDT ina SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. Thus, high piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters withappreciable bandwidth.

Pre-Formed Cavities

XBARs may be divided into two broad categories known as “the swimmingpool option” and the “backside etch option”. With the swimming pooloption, the piezoelectric plate is attached to a substrate and theactive portion of the piezoelectric plate floats over a cavity (the“swimming pool”) formed in the substrate, as shown in FIG. 3B and FIG.3C. With the backside etch option, the piezoelectric plate is attachedto a substrate and the active portion of the piezoelectric plate floatsover a void etched through the substrate from the back side (i.e. theside opposite the piezoelectric plate), as shown in the cross-sectionalviews of FIG. 1.

FIG. 5A is a view of a substrate 520 for an XBAR with a “swimming pool”cavity. The substrate 520 can be formed of a suitable material asdescribed above, such as silicon, sapphire, quartz, or some othermaterial or combination of materials. The substrate 520 has one or morecavities 540 (or trenches) pre-formed into the substrate 520, meaningthat the cavities are formed prior to the formation of the other partsof the device. These cavities 540 can be formed by any suitable method,such as etching with a selective etchant. Though four cavities are shownin FIG. 5A, more or fewer cavities can be formed in the substrate.

Alignment patterns 542 can also be formed in the substrate 520. In FIG.5A, the alignment patterns 542 are shown at the perimeter of thesubstrate 520, but the alignment patterns can be formed at any suitablelocation on the substrate that does not interfere with the cavities,such as at certain edges or between cavities. The alignment patterns maybe, for example, reticles or other shapes suitable for aligningphotomasks used in subsequent process steps.

The cavities 540 and the alignment patterns 542 are shown as trencheswith square shaped cross-sections, but these features can have anysuitable shape, such as trenches with slanted sides and slanted orrounded bottom surfaces. For example, the alignment patterns can beetched features defined by the same mask used to define the cavities,such that the alignment patterns and cavities can be formed in a singleprocess sequence. The alignment patterns can then facilitate alignmentof photomasks used to pattern the metal layers such that the IDT fingersare positioned over the cavities.

The substrate 520 can also be coated with SiO2, or another suitablecoating, on a surface of the substrate that will later be bonded with apiezoelectric plate. The surfaces of the cavities (i.e., the bottoms andsides of the cavities) may or may not be coated. Other surfaces of thesubstrate can be coated with the same or different materials, or leftuncoated.

FIG. 5B is a view of a piezoelectric slab 508 for forming apiezoelectric plate 510 for an XBAR. The piezoelectric slab 508 can beformed of any suitable material as described above, such as LiNbO3 andLiTaO3. Ions may be implanted into the back surface 511 of thepiezoelectric slab 508. The energy of the ion implantation determinesthe depth to which the ions are implanted. The implanted ions createdefects in the crystalline structure along the dashed line 509, whichfacilitates the wafer splitting. Thin piezoelectric slabs can also beproduced by the method of wafer polishing, in addition to ‘ion-slicing’.

FIG. 5C is a schematic plan cross-sectional view of the substrate 520 ofFIG. 5A bonded to the piezoelectric slab 508 of FIG. 5B. Thepiezoelectric slab 508 is bonded to the substrate 520 using a waferbonding process. The piezoelectric slab 508 may be aligned to thealignment pattern of the substrate 520.

A piezoelectric plate 510 can then be separated from the piezoelectricslab 508 at dashed line 509, as shown in FIG. 5D. In the ion-slicemethod, thermal shock or another suitable technique is used to fracturethe piezoelectric slab 508 along the defect plane at dashed line 509,leaving the piezoelectric plate 510 attached to the substrate 520. Oncethe piezoelectric plate 510 is separated, the separated surface can befurther polished via a suitable polishing method to prepare the surfacefor IDT formation, for example by chemical-mechanical polishing (CMP).

A conductor pattern including IDTs 530 is then formed on the separatedsurface, as shown in FIG. 5E. The IDTs are formed on the polishedsurface as described above. The photomasks used to define the conductorpattern are aligned to the alignment pattern. Each IDT 530 can bepositioned on the separated surface of the piezoelectric plate 510opposite a cavity 530. Final processing of the XBAR 500 then proceedwith no further etching required.

FIG. 6 is an example schematic circuit diagram and layout for a highfrequency band-pass filter 600 using XBARs. The filter 600 has a ladderfilter architecture including three series resonators 610A, 610B, 610Cand two shunt resonators 620A, 620B. The three series resonators 610A,610B, and 610C are connected in series between a first port and a secondport. In FIG. 6, the first and second ports are labeled “In” and “Out”,respectively. However, the filter 600 is symmetrical and either port andserve as the input or output of the filter. The two shunt resonators620A, 620B are connected from nodes between the series resonators toground. All the shunt resonators and series resonators are XBARs.

The three series resonators 610A, B, C and the two shunt resonators620A, B of the filter 600 are formed on a single plate 630 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 6, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 635). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

Description of Methods

FIG. 7 is a simplified flow chart showing a process 700 for making anXBAR or a filter incorporating XBARs. The process 700 starts at 705 witha substrate and a slab of piezoelectric material and ends at 795 with acompleted XBAR or filter. The flow chart of FIG. 7 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 7.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate as used in the previously presented examples. Thepiezoelectric plate may be some other material and/or employ some othercut angle. The substrate may preferably be silicon. The substrate may besome other material that allows formation of cavities by etching orother processing.

One or more cavities are formed in the substrate at 710, before thepiezoelectric slab is bonded to the substrate at 720. A separate cavitymay be formed for each resonator in a filter device. The one or morecavities may be formed using conventional photolithographic and etchingtechniques. Typically, the cavities formed at 710 will not penetratethrough the substrate, and the resulting resonator devices will have across-section as shown in FIG. 3A or FIG. 3B.

An alignment pattern can also be formed in the substrate. The alignmentpattern may be formed at the same time as the cavities, or before orafter the formation of the cavities. Photomasks used in further stepscan be aligned with the substrate via the alignment pattern.

Optionally, the substrate can be also be coated with SiO2 prior to beingbonded to the piezoelectric slab.

At 720, a back-side dielectric layer may be formed on the piezoelectricslab. If used, the back-side dielectric layer may be formed bydepositing one or more layers of dielectric material on the back side ofthe piezoelectric slab. The one or more dielectric layers may bedeposited using a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricslab. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric slab, such as only opposite wherethe interleaved fingers of the IDTs will be located. Masks may also beused to allow deposition of different thicknesses of dielectricmaterials on different portions of the piezoelectric slab.

At 730, the piezoelectric slab is bonded to the substrate. Thepiezoelectric slab and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric slab are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectric slaband the substrate. One or both mating surfaces may be activated using,for example, a plasma process. The mating surfaces may then be pressedtogether with considerable force to establish molecular bonds betweenthe piezoelectric slab and the substrate or intermediate materiallayers. The piezoelectric slab may be aligned to the alignment patternof the substrate.

At 740, a piezoelectric plate is separated from the piezoelectric slab,e.g., via ion beam wafer slicing. The newly exposed surface of thepiezoelectric plate opposite the substrate can then be polished inpreparation for formation of a conductor pattern.

A conductor pattern, including IDTs of each XBAR, is formed at 750 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate, where the IDTs are aligned with cavities onthe opposite side of the piezoelectric plate. The conductor layer maybe, for example, aluminum, an aluminum alloy, copper, a copper alloy, orsome other conductive metal. Optionally, one or more layers of othermaterials may be disposed below (i.e. between the conductor layer andthe piezoelectric plate) and/or on top of the conductor layer. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the conductor layer and the piezoelectricplate. A conduction enhancement layer of gold, aluminum, copper or otherhigher conductivity metal may be formed over portions of the conductorpattern (for example the IDT bus bars and interconnections between theIDTs).

The conductor pattern may be formed at 750 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 750 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 760, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In all variations of the process 700, the filter device is completed at770. Actions that may occur at 770 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 770is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at795.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface, wherein the substrate comprises a cavity opening tothe surface; a single-crystal piezoelectric plate having parallel frontand back surfaces, the back surface attached to the surface of thesubstrate except for a portion of the piezoelectric plate forming adiaphragm that spans the cavity; and an interdigital transducer (IDT)formed on the front surface of the single-crystal piezoelectric plate,wherein interleaved fingers of the IDT are aligned with the cavity suchthat the interleaved fingers are disposed on the diaphragm, the IDTconfigured to excite a primary acoustic mode in the diaphragm inresponse to a radio frequency signal applied to the IDT.
 2. The deviceof claim 1, wherein the primary acoustic mode is a shear acoustic mode.3. The device of claim 2, wherein a direction of acoustic energy flow ofthe primary acoustic mode is substantially orthogonal to the front andback surfaces of the diaphragm.
 4. The device of claim 1, wherein thesingle-crystal piezoelectric plate is one of lithium niobate and lithiumtantalate.
 5. The device of claim 1, wherein the substrate furthercomprises an alignment pattern in the surface that facilitates alignmentof the piezoelectric plate and the interleaved fingers.
 6. The device ofclaim 1, wherein the substrate comprises Si.
 7. The device of claim 6,wherein the substrate further comprises an SiO2 coating.
 8. The deviceof claim 1, further comprising: a front-side dielectric layer formed onthe front surface of the single-crystal piezoelectric plate over theIDT, wherein a resonant frequency of the primary acoustic mode isdetermined, in part, by a thickness of the front-side dielectric layer.9. The device of claim 1, further comprising: a back-side dielectriclayer formed on the back surface of the single-crystal piezoelectricplate over the IDT, wherein a resonant frequency of the primary acousticmode is determined, in part, by a thickness of the back-side dielectriclayer.
 10. A filter device comprising: a substrate having a surface,wherein the substrate comprises a plurality of cavities opening to thesurface; a single-crystal piezoelectric plate having parallel front andback surfaces, the back surface attached to the surface of thesubstrate, portions of the single-crystal piezoelectric plate forming aplurality of diaphragms spanning respective cavities in the substrate;and a conductor pattern formed on the front surface, the conductorpattern including a plurality of interdigital transducers (IDTs) of arespective plurality of acoustic resonators, wherein the conductorpattern is aligned with the cavities such that the respectiveinterleaved fingers of each of the plurality of IDTs are disposed on arespective one of the plurality of diaphragms, wherein all of the IDTsare configured to excite respective primary acoustic modes in therespective diaphragm in response to a respective radio frequency signalapplied to each IDT.
 11. The device of claim 10, wherein the primaryacoustic modes are shear acoustic modes.
 12. The device of claim 11,wherein a direction of acoustic energy flow of the primary acousticmodes is substantially orthogonal to the front and back surfaces of theplurality of diaphragms.
 13. The device of claim 10, wherein thesingle-crystal piezoelectric plate is one of lithium niobate and lithiumtantalate.
 14. The device of claim 10, wherein the substrate furthercomprises an alignment pattern that facilitates alignment of thepiezoelectric plate and the conductor pattern.
 15. The device of claim10, wherein the substrate comprises Si.
 16. The device of claim 14,wherein the substrate further comprises an SiO2 coating.
 17. The deviceof claim 10, further comprising: a front-side dielectric layer formed onthe front surface of the single-crystal piezoelectric plate over atleast one of the plurality of IDTs, wherein a resonant frequency of theprimary acoustic mode over the at least one of the plurality of IDTs isdetermined, in part, by a thickness of the front-side dielectric layer.18. The device of claim 10, further comprising: a back-side dielectriclayer formed on the back surface of the single-crystal piezoelectricplate over the IDT, wherein a resonant frequency of the primary acousticmode is determined, in part, by a thickness of the back-side dielectriclayer.
 19. A method of forming an acoustic resonator device, the methodcomprising: forming a cavity in a substrate, wherein the cavity is openinto a surface of the substrate, attaching a back surface of asingle-crystal piezoelectric plate having parallel front and backsurfaces to the surface of the substrate, wherein the back surface isattached to the surface of the substrate except for a portion of thepiezoelectric plate forming a diaphragm that spans the cavity; andforming an interdigital transducer (IDT) on the front surface of thesingle-crystal piezoelectric plate, wherein interleaved fingers of theIDT are aligned with the cavity such that the interleaved fingers aredisposed on the diaphragm, the IDT configured to excite a primaryacoustic mode in the diaphragm in response to a radio frequency signalapplied to the IDT.
 20. The method of claim 19, wherein the primaryacoustic mode is a shear acoustic mode.
 21. The method of claim 20,wherein a direction of acoustic energy flow of the primary acoustic modeis substantially orthogonal to the front and back surfaces of thediaphragm.
 22. The method of claim 19 further comprising forming analignment pattern in the surface of the substrate that facilitatesalignment of the piezoelectric plate and the interleaved fingers. 23.The method of claim 22 further comprising aligning the piezoelectricplate and the interleaved finger with the substrate via the alignmentpattern.
 24. The method of claim 19, wherein the substrate comprises Si.25. The method of claim 24 further comprising forming an SiO2 coating onthe substrate.
 26. The method of claim 19, wherein attaching a backsurface of the single-crystal piezoelectric plate further comprises:attaching a single-crystal piezoelectric slab to the surface of thesubstrate; and separating a portion of the single-crystal piezoelectricslab from the remainder of the single-crystal piezoelectric slab to formthe single-crystal piezoelectric plate, wherein the back surface isattached to the surface of the substrate except for a portion of thepiezoelectric plate forming a diaphragm that spans the cavity.